1. Field of the Invention
The present invention relates to electrical motor power systems and more particularly to those providing regenerative braking to a direct current motor.
2. Description of the Prior Art
In regenerative direct current motor controls, the braking of the motor necessary for precise speed regulation or other desired operating characteristics is obtained by applying the power generated in the motor during braking back to the active power source for the motor, such as a.c. supply lines.
Regenerative braking is advantageous in that, with proper control of armature current, braking may be done on a permanent basis, whereas resistive braking or other methods, such as plugging, are normally utilizable only in transient conditions or for isolated stops. Braking may also be accomplished very rapidly by regeneration.
Power may be regenerated or returned to the active motor power source by maintaining the same polarity of motor armature voltage while reversing the direction of armature current flow, as in "armature reversing" regenerative motor controls. Motor power may also be regenerated by maintaining motor armature current flow in the same direction while reversing the polarity of the armature voltage. Armature voltage reversal is obtained by reversing the motor field. A motor control employing this technique is termed a "field reversing" regenerative control and is the type to which the present invention is directed. A field reversing control includes a field circuit for reversing the motor field and an armature circuit for coordinately controlling the motor armature current.
The field circuit typically includes two groups of thyristors connected to an alternating current power source. The groups of thyristors control the direction and magnitude of the field current. The thyristors are controlled by a firing circuit which determines which group of thyristors is rendered conductive in accordance with the polarity of a motor operative condition error signal and determines the firing angle of the thyristors in accordance with the magnitude of the error signal.
In regenerative braking operation, the reversal of the motor field reverses the polarity of the motor flux and the armature counter e.m.f., assuming the direction of motor rotation remains instantaneously the same. The reversed counter e.m.f. biases thyristors in a thyristor bridge between the a.c. supply lines and the motor armature for current conduction in the same direction as during motoring anytime the counter e.m.f. is more negative than the a.c. supply lines voltage. This will include a portion of the negative half cycles of the alternating current power, thus permitting the reversal of voltage necessary for regeneration.
The reversal of the motor field winding current necessary for regeneration cannot be achieved instantaneously due to the inductance of the winding. This gives rise to a transient period in the operation of the regenerative motor control during which the motor is neither motoring nor regenerating but is essentially uncontrolled. Because of the uncontrolled state of the motor during this time interval, the precise speed regulation desired of a regenerative motor control is lessened.
To reduce the duration of the transient period, improved regenerative motor controls of the prior art have provided for reduction of the winding current by discharging the inductive energy of the motor field winding back to the motor field power supply. This is accomplished by firing the thyristors in the field circuit in the latter portions of each half cycle of the alternating current.
In the past, firing circuits for achieving such operation have generally utilized a pair of pulse generating circuits in the firing circuit for each group of thyristors. A first pulse generating circuit fires the thyristors during their conductive periods responsive to the magnitude of an input signal to the firing circuit derived from the motor operative condition error signal. The second pulse generating circuit responds to the alternating voltage as it approaches zero near the end of the conductive periods. The latter pulses produced by the latter circuit are used to discharge the field, since the inductive current of the field winding extends the conduction of the thyristors produced by the latter pulses into the negative half cycles of the alternating voltage, permitting regenerative discharge of the field to occur.
However, because the provision of the second or latter pulses depends upon line voltage, transients in this voltage may detrimentally affect the provision of these pulses.